operator_friend.cpp

#include <iostream>
using namespace std;

class Marks{
  int mark;
public:
    Marks(){
      mark = 0;
    }
    Marks(int m){
      mark = m;
    }
    void YourMarkPlease(){
      cout << "your mark is "<<mark<<endl;
    }

    void operator+=(int bonusmark){
      mark = mark + bonusmark;
    }

    friend void operator-=(Marks &curObj, int redmark);
};
// friend function;
// here we had to mention the marks object in parameter as it is not a member function
void operator-=(Marks &curObj, int redmark){
  curObj.mark -= redmark;
}

int main()
{
    Marks anilsmark(45);
    anilsmark.YourMarkPlease();

    int x = 20;

    anilsmark += x;
    anilsmark.YourMarkPlease();

    anilsmark -= x;
    anilsmark.YourMarkPlease();

    return 0;
}